Exhaust Gas Purification Device

ABSTRACT

An exhaust emission control device is realized which causes no battery exhaustion nor deterioration in engine startability even when the forced regeneration using the electric heater is conducted in the engine stopping status.  
     Disclosed is an exhaust emission control device in which adopted as a filter body  7  for capturing particulates is a heat-regenerative particulate filter  10  with electric heater, the captured particulates being burned by heating of the electric heater  9  to forcedly regenerate the particulate filter  10 . A control unit  37  responsible for a series of control operations with respect to the forced regeneration of the particulate filter  10  is constructed such that the forced regeneration in an engine stopping status is conduced only when the voltage of the battery  34  is higher than a stipulated value, and is temporarily interrupted when engine starting operation is conducted during the forced regeneration in the engine stopping status.

TECHNICAL FIELD

The present invention relates to an exhaust emission control devicewhich removes particulates from exhaust gas in an internal combustionengine such as a diesel engine.

BACKGROUND ART

Particulate or particulate matter from a diesel engine is mainlyconstituted by carbonic soot and soluble organic fraction (SOF) ofhigh-boiling hydrocarbon and contains a trace of sulfate (misty sulfuricacid fraction). In order to suppress such kind of particulates frombeing discharged to atmosphere, it has been envisaged as shown in FIG. 1that a particulate filter 4 is incorporated in an exhaust pipe 3 throughwhich exhaust gas 2 from a diesel engine 1 flows.

As shown in particular in FIG. 2, this particulate filter 4 comprises aporous honeycomb-structured filter body 7 made of ceramics such ascordierite and having lattice-like compartmentalized passages 5;alternate ones of the passages 5 have inlets plugged with plugs 8 andthe remaining passages with unplugged open inlets are plugged at theiroutlets with the plugs 8. Thus, only the exhaust gas 2 passing throughthin porous walls 6 compartmentalizing the respective passages 5 isdischarged downstream, and particulates are captured on inner surfacesof the walls 6.

The particulates in the exhaust gas 2, which are captured andaccumulated on the inner surfaces of the walls 6, are burned off throughspontaneous combustion when operation is shifted to an area with highexhaust temperature level. However, for example in a vehicle such as acity shuttle-bus tending to travel on congested roads, an engineoperation at or above a required, predetermined temperature level do notcontinue for a long period of time, so that a captured particle amountmay exceed a treated particulate amount, resulting in clogging of theparticulate filter 4.

To overcome this, it has been under review to provide an electric heaterso as to satisfactorily burn off particulates even in an engineoperation area with low exhaust temperature level. When heating ispositively carried out by this kind of electric heater, the particulatescan be burned off satisfactorily even in an engine operation area withlow exhaust temperature level.

A heat-regenerative particulate filter provided with an electric heaterhas been proposed for example in the following Patent Reference 1 in thename of the applicant of the present invention.

However, in such conventional proposals using an electric heater, muchelectricity is consumed for regeneration of the particulate filter 4 sothat required is a bulk power source which cannot be covered by anexisting battery for vehicles; the consumed electricity must becompensated for by the corresponding electric charge, disadvantageouslyresulting in lowering of mileage. There is also a fear that harmful gassuch as highly concentrated CO and HC may be generated and discharged tothe atmosphere since the particulates are burned off at a relatively lowtemperature level upon regeneration of the particulate filter 4.

Then, the inventor devised and filed a prior application (PatentReference 2 below) for an exhaust emission control device with aheat-regenerative particulate filter which is low in electricconsumption and discharges no harmful gas into atmosphere.

An underlying concept in the prior application is as shown in FIG. 3. Afilter body 7 made of silicon carbide and with a same structure as thatshown in the above with respect to FIG. 2 is provided at its front endwith an electric heater 9 such as a sheathed heater to thereby provide aheat-regenerative particulate filter 10. Arranged frontward and backwardof and in proximity to the particulate filter 10 are oxidation catalysts11 and 12, respectively, so as to obtain heat-retention effect on thefilter 10.

In such exhaust emission control device, the particulates captured bythe filter body 7 can be effectively burned off through heating of theelectric heater 9 even in an engine operation condition with low exhausttemperature level such as light-load operation; moreover, theheat-retention property of the front- and backward oxidation catalysts11 and 12 causes the filter body 7 to be promptly elevated intemperature into a situation for easy burn-off of the particulates. As aresult, the burn-off of the particulates can be completed in a shorterenergization time period than usual, thereby attaining substantialreduction in electric consumption.

Harmful gas such as highly concentrated CO and HC generated fromburning-off of the particulates through heating by the electric heater 9at a relatively low temperature level can be oxidized into anddischarged as harmless CO₂ and H₂O through its passage through therearward oxidation catalyst 12, whereby the harmful gas can bepreliminarily prevented from remaining in the exhaust gas 2 finallydischarged to the atmosphere.

Passage of the exhaust gas 2 through the frontward oxidation catalyst 11can convent NO, which occupies majority of NO_(x) in the exhaust gas 2,into highly reactive NO₂. As a result, the oxidation reaction of theparticulates can be substantially accelerated in the operationalcondition with relatively high exhaust temperature level, therebyfacilitating spontaneous combustion of the particulates without heatingby the electric heater 9, resulting in good burning-off.

-   [Reference 1] JP 62-255512A-   [Reference 2] JP 2005-090450A

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

Even if the heat-regenerative exhaust emission control device using theelectric heater 9 may be put to practical use through the meansillustrated above, it still an important issue that the forcedregeneration of the particulate filter 10 can be properly conducted ornot irrespective of the fact that engine is in motion or not especiallyin application to a vehicle having operation style with frequent enginestops such as a commercial vehicle used for parcel-delivery service orservice of delivering goods to convenience stores. The forcedregeneration using the electric heater 9 during the engine stoppingstatus will burden the battery extremely greatly and maydisadvantageously cause battery exhaustion or failure of engine startingduring forced regeneration in the engine stopping status.

The invention was made in view of the above and has its object toprovide an exhaust emission control device which causes no batteryexhaustion nor deterioration in engine startability even if the forcedregeneration using the electric heater is conducted during enginestopping status.

Means or Measures for Solving the Problems

The invention is directed to an exhaust emission control device whereinemployed is a heat-regenerative particulate filter comprising a filterbody for capturing particulates and an electric heater, the capturedparticulates being burned by heating through the electric heater forforced regeneration of the particulate filter, characterized in that acontrol unit responsible for a series of control operations for forcedregeneration of the particulate filter is constructed such that theforced regeneration in an engine stopping status is conducted only whenvoltage of a battery is higher than a stipulated value, and istemporarily interrupted when the engine starting operation is conductedin the engine stopping status.

Thus, when the battery voltage is lower than the stipulated value, theforced regeneration in the engine stopping status is not conduced norcontinued by the control unit. As a result, there is no fear thatbattery exhaustion is caused due to electricity consumption by theelectric heater in the engine stopping status.

When the engine starting operation is conducted during the forcedregeneration in the engine stopping status, the forced regeneration istemporarily interrupted by the control unit. As a result, the forcedregeneration and the engine starting operation which cause high electricburden are prevented from being conducted simultaneously in the enginestopping status, the engine starting operation being prioritized tocause the starting of the engine with no problem.

EFFECTS OF THE INVENTION

According to an exhaust emission control device of the inventionmentioned above, the following excellent effect will be obtained. Forcedregeneration of an particulate filter in engine stopping status is notconducted when voltage of a battery is lower than a stipulated value,and the forced regeneration is temporarily interrupted when an enginestarting operation is conducted during the forced regeneration in theengine stopping status, so that battery exhaustion and deterioration inengine startability can be preliminarily prevented even when the forcedregeneration of the particulate filter is conducted using the electricheater in the engine stopping status.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A schematic view showing a conventional particulate filterarranged in position.

[FIG. 2] A sectional view showing details of the particulate filter inFIG. 1.

[FIG. 3] A sectional view showing an underlying concept in a priorapplication using electric heater.

[FIG. 4] An overall schematic view showing an embodiment of theinvention.

[FIG. 5] A flowchart showing specific control procedures in a controlunit of FIG. 4.

EXPLANATION OF THE REFERENCE NUMERALS

-   7 filter body-   9 electric heater-   10 heat-regenerative particulate filter-   34 battery-   36 key switch-   37 control unit

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be described in conjunction with thedrawings.

FIGS. 4 and 5 show the embodiment of the invention in which partssimilar to those shown in FIGS. 1-3 are represented by the samereference numerals.

Shown in FIGS. 4 and 5 is an illustration of an exhaust emission controldevice of the invention comprising an exhaust pipe 3 divided halfwayinto two passages in which filter casings 13 and 14 are arranged inparallel with each other and in pair, a heat-regenerative particulatefilter 10 being accommodated in each of the filter casings 13 and 14.

The heat-regenerative particulate filter 10 illustrated is same as thatshown in FIG. 3 and comprises a filter body 7 made from silicon carbidewhich is provided at its front end with an electric heater 9 such as asheathed heater. Arranged in each of the filter casings 13 and 14 andfront- and rearward of and in proximity to the particulate filter 10 areoxidation catalysts 11 and 12 for heat-retention effect on theparticulate filter 10.

The front- and rearward oxidation catalysts 11 and 12 shown areflow-through type ones each comprising a honeycomb-structured carriermade of ceramics such as cordierite and carrying a proper amount ofplatinum; a volume and a carried platinum amount of the rearwardoxidation catalyst 12 may be decreased than those of the frontwardoxidation catalyst 11.

Each of the filter bodies 7 of the filter casings 13 and 14 mayintegrally carry an oxidation catalyst so as to accelerate the oxidationreaction of the particulates captured by the filter body 7.

Furthermore, exhaust shutters 15 and 16 are respectively arranged oninlet sides of the filter casings 13 and 14 so as to selectively directthe exhaust gas 2 flowing through the exhaust pipe 3 into either of thefilter casings 13 and 14, the exhaust shutters 15 and 16 being adaptedto be selectively opened and closed by air cylinders 20 and 21 which inturn are driven by pressurized air from an air tank 17 viaelectromagnetic valves 18 and 19, respectively.

Inserted and arranged just downstream of the exhaust shutters 15 and 16are air induction pipes 25 and 26 which guide, via electromagneticvalves 23 and 24, combustion air taken in from atmosphere by driving anair pump 22 to inlets of the filter casings 13 and 14, respectively.

In the drawings, reference numerals 27 and 28 denote pressure sensors;29 and 30, temperature sensors; 31 and 32, relays for the heaters; 33, arelay for the pump; 34, a battery; 35, an automatic regeneration switch;36, a key switch; and 37, an electronic control unit (ECU) which servesalso as an engine controlling computer. Inputted to the control unit 37are detection signals from the pressure sensors 27 and 28 andtemperature sensors 29 and 30 and on-off signals from the automaticregeneration switch 35 and key switch 36; outputted from the controlunit 37 are control signals to the relays 31 and 32 for the heaters,relay 33 for the pump, electromagnetic valves 18, 19, 23 and 24. Thus, aseries of control operations referred particularly to hereinafter areconducted for forced regeneration of the particulate filter 10.

For example, as shown in FIG. 4, when the running or driving is carriedout with exhaust shutters 16 and 15 being respectively closed and openedfor flow of the exhaust gas 2 only into the filter casing 13, NOoccupying majority of NO_(x) in the exhaust gas 2 flowing into thefilter casing 13 is converted into highly reactive NO₂ upon passing ofthe exhaust gas 2 through the frontward oxidation catalyst 11, wherebythe oxidation reaction of the particulates is substantially facilitatedinto sufficient burning-off in the operational condition with theexhaust temperature level at or above about 250° C. However, when alight-load operational condition with temperature level greatly fallingbelow 250° C. is continued for a long period of time, for example,during running or driving on congested city roads, satisfactory burn-offof the particulates cannot be expected.

To overcome this, when excess of an accumulated particulate amount inthe filter body 7 in the filter casing 13 over a predetermined amount isestimated on the basis of the detected values of the pressure sensors 27and 28 (estimation is based on pressure difference between the enteringand discharge sides of the filter body 7), by the control unit 37 thevalve 18 is opened to close the shutter 15 and the valve 19 is closed toopen the shutter 16, whereby the exhaust gas 2 is bypassed to the otherfilter casing 14 into a situation that the filter body 7 in the filtercasing 13 is not exposed to the flow of the exhaust gas 2; then, thecontact point of the relay 31 for the heater is closed to energize andheat the electric heater 9 in the filter casing 13, whereby the filterbody 7 in the filter casing 13 is positively heated.

When the exhaust temperature detected by the temperature sensor 29reaches a predetermined value due to such positive heating of the filterbody 7 by the electric heater 9, by the control unit 37 the relay 33 isclosed to drive the air pump 22 and the valve 23 is opened to introducethe combustion air from the pump 22 into the inlet of the filter casing13 via the pipe 25.

When the exhaust gas 2 is bypassed in this manner and the heating by theelectric heater 9 is continued with the combustion air guided to theinlet of the filter casing 13, heat due to the electric heater 9 iseffectively imparted to the filter body 7 without being robbed of by theexhaust gas 2, whereby the filter body 7 is effectively heated topromote the oxidization reaction of the particulates captured by thefilter body 7, and oxidizing atmosphere around the filter body 7 iselevated for easy burn-off of the captured particulates.

In this case, the heat-regenerative particulate filter 10 is sandwichedand heat insulated by the front- and rearward oxidation catalysts 11 and12 in one and the same filter casing 13, 14, so that the beginning ofthe oxidization reaction of the captured particulates causes the filterbody 7 to be rapidly elevated in temperature, whereby the particulateshave tendency of being more easily burned off; as a result, theparticulates can be completely burned off with a time period ofenergization shorter than that they can conventionally, resulting inreduction of the electric power consumption in comparison with the priorart.

The harmful gas such as highly concentrated CO or HC generated due toburn-off of the particulates with relatively low temperature level bythe heating of the electric heater 9 is oxidized into harmless CO₂ orH₂O during passage through the rearward oxidation catalyst 12 and isdischarged.

In a course of operations for a long period after completion of theregeneration in the filter body 7 in the one filter casing 13, theaccumulated particulate amount in the filter body 7 in the other filtercasing 14 may be estimated to excess the predetermined amount; then, theregeneration of the filter body 7 with respect to the other filtercasing 14 may be conducted in the same manner as the above.

More specifically, by the control unit 37 the valve 18 is closed to openthe shutter 15 and the valve 19 is opened to close the shutter 16,whereby, in this time, the exhaust gas 2 is bypassed to the one filtercasing 13. The relay 32 is closed to energize and heat the electricheater 9 in the filter casing 14 to positively heat the filter body 7 inthe filter casing 14; thereafter, when the exhaust temperature detectedby the temperature sensor 30 reaches a predetermined value, by thecontrol unit 37 the contact point of the relay 33 is closed to drive theair pump 22 and the valve 24 is opened to introduce the combustion airfrom the pump 22 into the inlet of the filter casing 14 via the pipe 26.

The control unit 37 responsible for the series of control operationswith respect to the forced regeneration of the particulate filter 10 asmentioned in detail in the above has a function that the forcedregeneration in the engine stopping status is conducted only in thecondition that the voltage of the battery 34 is higher than theprescribed value, and the forced regeneration is temporarily interruptedwhen engine starting operation is conducted by turning on the key switch36 during the forced regeneration in the engine stopping status.

More specifically, specific control procedures in the control unit 37with respect to the above-mentioned stop and interruption of the forcedregeneration in the engine stopping status are as shown in the flowchartof FIG. 5. Firstly, in step S1, it is determined whether the forcedregeneration of the particulate filter 10 is required or not. Thedetermination is repeated until determination results is affirmative onnecessity of the forced regeneration.

For the determination whether the forced regeneration is required ornot, an accumulated particle amount in the filter body 7 is estimated onthe basis of detected values of the pressure sensors 27 and 28 (pressuredifference between inlet and outlet sides of the filter body 7: pressureloss); when the estimated value exceeds a predetermined value, then itis determined that the forced regeneration is required. As mentionedbelow, the accumulated particle amount may be alternatively estimatedfrom the operational condition of the engine.

The control unit 37 in the embodiment serves also as an engine controlcomputer and is responsive also to fuel injection control to respectivecylinders based on engine revolution and accelerator stepped-in degree,so that the fuel injection amount which is evident as of decision of thefuel injection control is extracted to estimate a basic particulategeneration amount in the current engine-operating state depending uponsuch injection amount and revolution, using a particulate generationamount map. Then, the basic, estimated particulate generation amount ismultiplied by a correction factor applied in consideration of variousparameters on generation of the particulates and then a treated particleamount in the current engine operating state is subtracted therefrom todetermine a captured particle amount in the particulate filter 10. Suchcaptured amount is momentarily submitted to a cumulative computation toestimate an accumulated particulate amount

There have been various concepts for estimating such accumulatedparticulate amount; other methods than is illustrated above may be, ofcourse, employed for estimation of the accumulated particulate amount.

When it is determined in step S1 that the forced regeneration of theparticulate filter 10 is required, the routine proceeds to step S2 whereit is determined whether the engine is in stopping status or not; whenthe engine is in motion, the routine proceeds to step S3 where conductedare a series of control operations with respect to the forcedregeneration (switch-on and temperature control of one of the electricheaters 9, switch-on of the air pump 22, passage changeover by theexhaust shutters 15 and 16, changeover on where combustion air is to beintroduced, and the like).

After the forced regeneration is conducted in step S3, it is monitoredin step S4 whether the engine stopping operation of switching off thekey switch 36 is conducted or not during the forced regeneration. Whenthe forced regeneration is continued with no engine stopping operation,the flow proceeds to step S5 where it is determined whether theregeneration of the particulate filter 10 (burn-off of the capturedparticulates) is completed or not. When it is determined to becompleted, then the series of control operations with respect to theforced regeneration are ended. While it is not determined that theregeneration of the particulate filter 10 is completed, the proceduresfrom step S3 are repeated.

As to the determination whether or not the regeneration of theparticulate filter 10 is completed in step S5, for example, it ismonitored how many minutes the condition of the measured value of thetemperature sensor 29 or 30 exceeding a predetermined value iscontinued; when a total of the minutes determined exceeds apredetermined period of time enough for burn-off of the estimatedaccumulated particulate amount, it may be determined to be completed.

Alternatively, a treated or burn-off particulate amount per unit timemay be estimated on the basis of the measured value of the temperaturesensor 29 or 30, such treated or burn-off amount being submitted to acumulative computation whenever unit time elapses; then, the completionis determined when the estimated, accumulated particulate amount in theparticulate filter 10 is equal to the cumulated value of the treated orburn-off particulate amount.

When the engine stopping operation is conducted during the forcedregeneration in step S4, the routine proceeds to step S6 where it isascertained whether the automatic regeneration switch 35 is on-state ornot; when the switch 35 is not on-state, the procedures from step S2 arerepeated.

The automatic regeneration switch 35 is a switch through which a drivercan freely interrupt the forced regeneration by his/her will in a casewhere automatic execution of the forced regeneration in the enginestopping status is to be averted (for example, in indoor parking).

When on-state of the automatic regeneration switch 35 is ascertained instep S6, it is determined in step S7 whether the voltage of the battery34 is higher than a stipulated value or not; only when the voltage ofthe battery 34 is determined to be higher than the stimulated value, theroutine proceeds to step S8 where the series of control operations withrespect to the forced regeneration are conducted.

When the voltage is the battery 34 is lower than the stipulated value,then routine returns from step S7 to step S2 where repeated is thedetermination whether the engine is in stopping status or not; it isactually a status of awaiting the engine starting operation forprocession to step S3.

After the forced regeneration is conducted in step S8, it is monitoredin step S9 whether the engine starting operation of switching on the keyswitch 36 is conducted or not during the forced regeneration. In a casewhere the forced regeneration is continued without the engine startingoperation, the routine proceeds to step S10 where it is determinedwhether the regeneration of the particulate filter 10 (burn-off of thecaptured particulates) is completed or not; when the completion isdetermined, the series of control operations with respect to the forcedregeneration are ended; while it is not determined that the regenerationof the particulate filter 10 is completed, the procedures from step S6are repeated. The description on the determination of the regenerationof the particulate filter 10 being completed is omitted since it is thesame as that disclosed with respect to step S5.

When the engine starting operation is conducted during the forcedregeneration in step 9, the routine proceeds to step S11 where theforced regeneration is temporarily interrupted for about 10 seconds orso, thereafter the routine returns to step S2 where the similarprocedures are repeated.

More specifically, normally, the engine will start through the enginestarting operation within the interruption time of about 10 seconds orso, so that, after the engine starting operation is ascertained in stepS2, the routine proceeds to step S3 where the forced regeneration isconducted in the engine operating status.

By controlling the forced regeneration of the particulate filter 10through the control unit 37 as mentioned above, the forced regenerationin the engine stopping status is not conducted (continued) by thecontrol unit 37 when the voltage of the battery 34 is lower than thestipulated value, thereby preliminarily preventing the batteryexhaustion through electric consumption of the electric heater 9 in theengine stopping status; moreover, when the engine starting operation ofswitching on the key switch 36 is conducted during the forcedregeneration in the engine stopping status, the forced regeneration istemporarily interrupted by the control unit 37. As a result, the forcedregeneration using the electric heater 9 and the engine startingoperation which cause high electric burden are prevented from beingconducted simultaneously in the engine stopping status, the enginestarting operation being prioritized to cause the starting of the enginewith no problem.

Thus, according to the above-mentioned embodiment, the forcedregeneration is not conducted in the condition that the voltage of thebattery 34 is lower than a stipulated value. Moreover, the forcedregeneration is temporarily interrupted when the engine startingoperation is conducted during the forced regeneration in the enginestopping status. As a result, battery exhaustion and deterioration inengine startability can be preliminarily prevented even when the forcedregeneration of the particulate filter 10 is conducted using theelectric burner 9 in the engine stopping status.

It is to be understood that the exhaust emission control deviceaccording to the invention is not limited to the above-mentionedembodiment and that various changes and modifications may be madewithout leaving the spirit of the invention. For example,heat-regenerative particulate filters may not be in parallel with eachother and in pair; for example, they may be replaced by providing asingle heating type particulate filter and a bypass passage bypassingthe same. The filter body may not carry an oxidation catalyst. Thefilter body may not be of a shape shown in the figures. The electricheaters may be fitted to the filter bodies in a manner different fromthat shown in the figures. The engine starting operation may not belimited to switch-on of key switch.

1. An exhaust emission control device wherein employed is aheat-regenerative particulate filter comprising a filter body forcapturing particulates and an electric heater, the captured particulatesbeing burned by heating through the electric heater for forcedregeneration of the particulate filter, comprising a control unitresponsible for a series of control operations for forced regenerationof the particulate filter being constructed such that the forcedregeneration in an engine stopping status is conducted only when voltageof a battery is higher than a stipulated value, and is temporarilyinterrupted when the engine starting operation is conducted in theengine stopping status.